if you are allowed to do it yourself, it is very reasonable, but if you are part of a buy sell through a broker or bank (are there loans?) they may not allow you to do it yourself and your estimate is quite fair for a firm to do it. Just make sure you get traveling papers in hand and a paid receipt for the work.
IN the mid atlantic area it runs from a low of about $250 to over $500 depending on the firm and the boat being documented. Keep in mind you can not document a boat from Canada or elsewhere in most instances.
I bought my boat through a broker and had a loan, but was still able to do the paperwork myself. Of course, the boat was already documented and this was a change in ownership, so that might make a difference. If yours is a first-time, it will cost you $133. I would certainly try to do it myself before forking over $350.